As mobile terminals such as a cellular phone and a note-type personal computer have become widespread, a battery as their power source has become more important. Such a secondary battery must be small and light-weight While having a higher capacity and must exhibit a property that it is tolerant to degradation due to repetitive discharging and charging.
Lithium metal is sometimes used as an anode for such a secondary battery in the light of its higher energy density and light weight. There is, however, a problem that as a charge/discharge cycle is repeated, needle crystals (dendrites) are precipitated on a lithium surface during charge, and these crystals penetrate a separator to cause internal short-circuit, leading to a reduced battery life. When using a carbon material such as graphite and hard carbon capable of occluding and releasing lithium ions as an anode, a discharge-charge cycle can be successfully repeated. However, a graphite material may have a smaller capacity than lithium metal or a lithium alloy and thus have a larger irreversible capacity during the initial charge/discharge, leading to a lower charge/discharge efficiency and thus to a smaller energy density.
Recently, as a material for increasing an energy density, the use of Si as an anode activator has been investigated. It is believed that such an anode material may be used to provide a high capacity anode. An anode containing this type of Si exhibits a larger lithium occlusion/release amount per a unit volume and has a higher capacity. However, during occlusion or release of lithium ions, the electrode activator itself expands or shrinks, causing pulverization, and thus has a larger irreversible capacity during the initial-charge/discharge and a shorter charge/discharge cycle life.
For preventing pulverization of an anode, there has been reported the use of an Si oxide as an activator (Patent document 1). In the report, there has been observed that an Si oxide can be used as an activator to improved cycle properties because of reduction in a cubic expansion/shrinkage per a unit weight of the activator. There has been, however, a problem that the oxide is less conductive and thus less electro-collecting, leading to a larger irreversible capacity.
Patent document 1: Japanese Patent No. 2,997,741.